Starch-hydrolyzed polyacrylonitrile graft copolymers exhibiting the capacity to absorb from about 300 to 1000 times their weight of deionized water are known at this time. The development of these compositions was carried out by the Northern Regional Research Laboratory, Peoria, Ill. The starch-hydrolyzed polyacrylonitrile graft copolymer is produced by exposure of starch, either gelatinized or ungelatinized, to a catalyst such as ceric ammonium nitrate which acts as a catalyst to generate free radicals in the starch chain. Polyacrylonitrile chains become attached to these free radicals by copolymerization. A wide range of substitution in these copolymers is known in the art. For example, U.S. Pat. No. 3,035,099 shows the preparation of copolymers in which the starch to polyacrylonitriles molar ratios range from 1:1.5 to 1:9. The variations in molar ratio of the components of the copolymer is not critical to the practice of this invention. The resulting material is then saponified in sodium hydroxide to hydrolyze the polyacrylonitrile chains to carboxy amide and alkali metal carboxylate groups mixed with metal salts. Drying the hydrolyzed material can be accomplished by tumble air drying or vacuum drying. After drying, the material can absorb about 300 to 400 times it weight. Washing the copolymer before drying with alcohol increases its absorbency to 800 to 1000 times its weight.
The copolymer can be made as film, flakes, powder or mat. These forms take up water, swelling but not dissolving and hold it in expanded duplications of their own dry shapes. Films extend and thicken in all dimensions. Powders become piles of water textured like crushed ice. A flake expands to a clear, angular piece of water. The swollen forms shrink in dilute acid, expand again in dilute alkali solution. They also shrink as they dry and expand again when absorbing water.
The copolymer, with these properties, can be mixed with or coated on a wide variety of materials including, for example, sand, straw, sawdust, seeds and roots, natural or synthetic fibers, flour, gelatin, and starch. It can hold water in soils, animal bedding and kitty litter, toweling and diapers, bandages, surgical pads, and dental absorbents.
Some of the major problems in using the copolymer is that the dry copolymer films are extremely brittle and adhere poorly to substrates. The copolymer in powder form is difficult to hold in position.